The cams which drive air intake and exhaust valves of an engine are designed such that the valve timing is optimized with respect to the characteristics required by the engine.
This optimum valve timing however depends on the running condition of the engine. On low load, for example, a small valve lift amd valve opening period are required, while on high load a large valve lift and valve opening period are required. In particular, in the case of automoble engines which have a wide range of running conditions it is impossible to have optimum matching in all cases, and it is therefore by no means easy to set the valve timing to suit the running conditions.
In Tokkaisho 63-167016 published by the Japanese Patent Office, a variable cam engine is proposed wherein several cams with different shapes are provided, and the optimum valve timing is obtained by selecting these cams depending on the engine running conditions.
This engine is designed to give high power over the whole range of speeds, i.e. from low speed to high speed, by changing from a low speed cam which gives hhigh torque at low speed to a high speed cam which gives high torque at high speed or vice versa depending on the running conditions.
In order that the engine output torque does not vary discontinuously, this change-over between cams is made at a certain engine speed chosen such that the output torques of the cams is the same for the same throttle opening.
However, if for example two cams are provided having shapes which respectively maximize the power (torque) at low speed and high speed (power cams), and another can is provided to improve fuel comsumption performance on partial load (economy cam), there is still a very large difference of output torque for a given throttle opening when a change-over is made from the economy cam to the power cams, or vice versa.
This is due to the fact that there is no engine speed at which the output torques of the economy cam and the power cams conincide for the same throttle opening.
To solve this problem, the torques can be made to coincide by having an arrangement wherein the throttle valve opening can be controlled independently of the accelerator pedal, and the throttle opening or ignition timing are automatically compensated by an amount sufficient to absorb the torque step as judged from the throttle opening or engine speed when the change-over is made. When changing over from the economy cam to the power cams, for example, the torque increases rapidly if the throttle opening is not changed, so the throttle valve opening is reduced or the ignition timing is temporarily retarded.
However, depending on the running conditions when the change-over is made, the amount by which the throttle opening has to be compensated may be extremely large. Further, however rapidly the compensation is made, there will be a response delay until the torque reaches the target value. It is thus impossible to avoid a torque shock when the change over is made.